Aircraft, in particular passenger aircraft, comprise a large number of electric loads which require electrical power for the operation thereof. For this purpose, modern aircraft comprise an on-board electrical system. European patent EP 1 928 745 B1 discloses such an energy supply system in an aircraft. The present invention and its underlying problem will hereinafter be described on the basis of an on-board power supply system for an aircraft, however, without restricting the invention to this sort of application. It shall be most understood that an on-board power supply system comprises both an AC and DC power supply, whereas the present invention is directed to the operation of a DC voltage.
A major goal of the aviation industry is the reduction of emissions in combination with cost efficiency. On the one hand eco-friendly aircraft can result in cost savings because of the forecasted rising fuel prices in the future. On the other hand environmental pollution is in the center of actual public discussions. Therefore, one major issue in the development of modern aircraft is the reduction of weight which has a direct impact on reduced fuel consumption. During normal operation of an aircraft, the on-board electrical systems use only 0.2% of the whole energy provided by the aircraft engines; hence savings in the electricity consumption have only a small impact on the aircraft's fuel consumption. However, with the “More Electric Aircraft”-concept the complexity of the on-board electrical system is increasing and consequently also the weight of their electric wiring. In conclusion saving weight has a huge impact on eco-efficiency and fuel consumption.
The present patent application refers to the use and integration of HVDC in the on-board power supply system of modern aircraft. Modern on-board electric power supplies use so called HVDC (High voltage DC). HVDC enables DC output voltages of +/−135 V, 270V/0V and +/−270V depending on the engine generators and the driving circuitry. HVDC based power supply in an aircraft is in particular advantageous with regard to weight reduction approaches, however, the implementation of HVDC within the power supply system is not trivial and needs some additional effort.
Modern aircraft with HVDC comprise a 3-phase on-board electrical system having a supply voltage of 115 Volt or 230 Volt. The electrical power is generated by engine generators which in turn are driven by the engines of the aircraft. The engine generators generate an alternating voltage (AC) of variable frequency. The AC frequency can vary in a relatively wide frequency range of for example 360-800 Hz as a function of the rotational speed of the engine. Earlier on-board electrical system provided a constant frequency of 400 Hz which was generated via a so-called constant speed drive (CSD). A constant speed drive (CSD) is a mechanical gearbox that takes an input shaft rotating at a wide range of speeds, delivering this power to an output shaft that rotates at a constant speed, despite the varying input. They are used to drive alternating current (AC) electrical generator that require a constant input speed.
Bigger loads are connected to all three phases of the power supply, whereas smaller loads are typically connected only between one phase and the aircraft chassis which allows avoiding the weight consuming return conductor. In order to provide a more efficient, especially weight efficient on-board power supply, it may be possible to increase the AC voltage provided by the generator. This way, the weight may be reduced. However, in all energy supply concepts and especially with a HVDC power supply the existing airport and airline infrastructure has to be taken into account too. An aircraft has two possibilities to supply the electric loads while the engines are switched off:
The first one is using the traditional auxiliary power unit (APU). The problem when using the APU is an efficiency of only 8%, comparably high kerosene consumption and a high level of noise emission. Because of that the use of the APU is not allowed at some airports. Therefore, in modern aircraft a multifunctional fuel cell system may be used in order to reduce pollution during ground operation. However, integrating such a fuel cell system would make an adaption of the primary aircraft grid necessary, such as the introduction of a high voltage DC (HVDC) level.
The second possibility is to supply electric energy to the aircraft via external systems such as ground power units (GPU). A GPU is a type of auxiliary power unit (APU) used on the ground at airports to supply electric power toward aircraft on the ground, to sustain interior lighting, ventilation and other requirements before starting of the main engines or the aircraft auxiliary power unit (APU). This may be a self-contained engine-generator set, or it may convert commercial power from the local electrical grid to the voltage and frequency needed for the aircraft. A standardized GPU allocates 115 V AC with a fixed frequency of 400 Hz and a maximum power of 90 kVA per unit. In order to connect the GPU to the aircraft, aircraft have a standardized connector to feed the electrical system externally. At that connector mobile APU-cars or an inverter, fed by the airport grid, can be connected.
There are different approaches to implement a HVDC based aircraft power supply, each with specific benefits, however, also with some needs especially with regard to the implementation of the different voltage levels of the HVDC. In one known approach, which is used in Boeing's 787 (Aero Quarterly QTR_04/07, Mike Sinnett, “787 No-Bleed Systems: Saving Fuel and Enhancing Operational Efficiencies”, pages 6-11), the 230 Volt AC voltage which is generated by the engine generators 1 of the aircraft is rectified in a rectifier circuit 2. This generates a ±270 Volt supply voltage at the DC output. Consequently, the loads 3 (without any voltage divider means) have to be laid out for a supply voltage of 540 Volt. The aircraft chassis 4 (fuselage) is connected to the airport earth 5 which forms the common ground or reference potential of the aircraft and the GPU 6. In this approach, no reverse connector is needed since this function is fulfilled by the grounded aircraft body. However, with this approach additional transformers 7 are needed in order to couple the 115 Volt powered GPU 7 to the on-board HVDC of the aircraft.